Classification of Massive Star Magnetospheres

A Magnetic Confinement vs. Rotation Classification of Massive-Star Magnetospheres

Abstract

Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement (which sets the Alfvén radius ) and stellar rotation (which sets the Kepler co-rotation radius ) provides a useful organisation of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with  kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of vs. . This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with , vs. dynamical magnetospheres (DM), with . We show how key observational diagnostics, like the presence and characteristics of H emission, depend on a star’s position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with H emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall timescale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown timescales. Finally, we discuss how other observational diagnostics, e.g. variability of UV wind lines or X-ray emission, relate to the inferred magnetic properties of these stars, and summarise prospects for future developments in our understanding of massive-star magnetospheres.

keywords:
stars: magnetic fields – stars: early-type –circumstellar matter – stars: mass-loss– stars: rotation– stars: fundamental parameters – stars: emission-line, Be – ultraviolet: stars – X-rays: stars.
12

1 Introduction

Building on pioneering detections of strong (kG) fields in the chemically peculiar Ap and Bp stars (e.g. Babcock, 1947; Borra & Landstreet, 1980), new generations of spectropolarimeters have directly revealed large-scale, organised (often predominantly dipolar) magnetic fields ranging in dipolar strength3 from order of 0.1 to 10 kG in several dozen OB stars (e.g. Donati et al., 2002, 2006a; Hubrig et al., 2006; Petit et al., 2008; Grunhut et al., 2009; Martins et al., 2010). In recent years, an observational consortium known as MiMeS (for Magnetism in Massive Stars) has been carrying out surveys to detect new magnetic OB stars, while also monitoring known magnetic OB stars with high resolution spectroscopy and polarimetry (Wade et al., 2011a). Concurrently, theoretical models (Townsend et al., 2005, 2007) and magnetohydrodynamical (MHD) simulations (ud-Doula & Owocki, 2002; ud-Doula et al., 2008, 2009) have explored the dynamical interaction of such fields with stellar rotation and mass loss, showing for example how suitably strong fields can channel the radiatively driven stellar wind outflow into a circumstellar magnetosphere. This paper aims now to provide an initial classification of the observed magnetospheric properties for a broad sample of magnetic massive stars.

The idea of a magnetosphere has been exploited to explain particular properties of some massive stars, for example the photometric light curve and H variations of the He-strong star  Ori E (Landstreet & Borra, 1978), the UV resonance line variations of magnetic Bp stars (Shore & Brown, 1990), the X-ray properties of the O-type star  Ori C (Gagné et al., 2005), and the radio emission of Ap-Bp stars that correlates with the field strength (Linsky et al., 1992).

For a few specific stars, previous work has already shown some promising agreement between theoretical predictions and key observational characteristics. For example, the luminosity, hardness, and rotational modulation of X-rays observed in the O-type star  Ori C all match well the X-rays computed in MHD simulations of magnetically confined wind shocks, which result from the collision of the wind from opposite footpoints of closed magnetic loops in its  kG dipole field (Gagné et al., 2005). In the B2p star  Ori E, the combination of its very strong ( kG) field and moderately fast (1.2-day period) rotation leads to formation of a centrifugally supported magnetosphere with observed, rotationally modulated Balmer line emission reasonably well explained within the Rigidly Rotating Magnetosphere model (RRM; Townsend & Owocki, 2005; Townsend et al., 2005). Most recently, Sundqvist et al. (2012) showed that, even in the very slowly rotating (537-day period) O-type star HD 191612, the magnetic confinement and transient, dynamical suspension of its strong wind mass loss leads to sufficient density to likewise match the observed rotationally modulated Balmer line emission.

Building on these results, along with those from the MiMeS observational survey, this paper compiles an exhaustive list of confirmed magnetic, hot OB stars, along with their physical, rotational and magnetic properties (§2). As a basis for organising this compilation according to modelling predictions, we follow (§3) the two-parameter theoretical study of ud-Doula et al. (2008), which characterised MHD simulation results according to the strength of wind magnetic confinement () and fraction of stellar rotation to orbital speed at the stellar equatorial radius (). These dimensionless parameters uniquely define associated characteristic radii, namely the Alfvén radius and Kepler co-rotation radius .

We show in particular (§4) that an associated log-log plot of known magnetic stars in the -vs.- (or equivalently -vs.-) plane, the magnetic confinement-rotation diagram, provides a particularly useful initial classification for interpreting the H properties of their associated magnetospheres. Furthermore, we also explore the UV and X-ray characteristics as potential additional proxies of magnetospheres (§5). We briefly review our main findings and conclusions in §6.

2 Exhaustive list of magnetic O-type and early B-type stars

A central goal of this paper is to compile a comprehensive list of OB stars for which magnetic fields have been convincingly detected via the Zeeman effect, so that their magnetospheres can be classified.

The work here is done within the context of the MiMeS project (Wade et al., 2011a), which aims to expand the population of known magnetic stars, confirm the detection of poorly studied magnetic OB stars, and provide a modern determination of their magnetic field characteristics. These goals are being achieved through Large Program observing allocations at the Canada-France-Hawaii Telescope (CFHT), the Télescope Bernard-Lyot (TBL) and the ESO 3.6m Telescope to collect high resolution, high signal-to-noise ratio spectropolarimetric observations of massive stars (see Wade et al., 2011a; Oksala et al., 2012; Alecian et al., 2011, respectively).

Table 1 lists our derived sample of 64 magnetic stars, ordered by spectral type and temperature. Column (1) gives the numerical identification (ID) we use in the figures. Column (2) gives the HD number or a SIMBAD-friendly4 designation. A dagger indicates that a note for that particular star is available in Appendix A. Columns (3) and (4) give a commonly used designation and the spectral type, respectively. Column (5) indicates if the star is a known single- or double-line spectroscopic binary (SB1/2), slowly pulsating B-type star (SPB),  Cep-type pulsator ( Cep), or a Herbig Be star (HeBe). Table 2 compiles, for each star, the list of references where information can be found or how it is derived from MiMeS observations or other archival data.

ID Star Alt. name Spec. type Remark
kK cgs d km  s kG
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
1 HD 148937 O6 f?p 15 60 7.0323  45
2 CPD -28 2561 O6.5 f?p 14 43 70
3 HD 37022   Ori C O7 Vp SB1 9.9 45 15.424 24
4 HD 191612  O6 f?p-O8 fp SB2 14 30 537.2  60
5 NGC 1624-2 O6.5 f?cp-O8 f?cp 9.7 34 158.0  3
6 HD 47129  Plaskett’s star O7.5 III SB2 10 56 305
7 HD 108 O8 f?p 19 43 18000  50
8 ALS 15218  Tr16-22 O8.5 V 9.0 28 25
9 HD 57682 O9 V 7.0 17 63.571 15
10 HD 37742   Ori Aa O9.5 Ib SB2 25 40 7.0 110
11 HD 149438  Sco B0.2 V 5.6 11 41.033 5
12 HD 37061  NU Ori B0.5 V SB2 5.7 19 225
13 HD 63425 B0.5 V 6.8 17  10
14 HD 66665 B0.5 V 5.5 9.0 21  10
15 HD 46328  CMa B1 III  Cep 8.6 9.0 4.26  15
16 ALS 8988 NGC 2244 OI 201 B1 HeBe 4.7 12 23
17 HD 47777 NGC 2264 83 B1 III HeBe 5.0 9.0 65
18 HD 205021   Cep B1 IV SB2, Cep 6.5 12 12.00092 27
19 ALS 15211  Tr16-13 B1 V 4.9 9.0
20 HD 122451   Cen B1 SB2, Cep 8.7 8.8 75
21 HD 127381  Lup B1/B2 V 4.8 9.0 3.0197 68
22 ALS 3694 NGC 6193 17 B1 5.6 11 83
23 HD 163472 V 2052 Oph B1/B2 V  Cep 4.1 10 3.638833 68
24 HD 96446  V 430 Car B1 IVp/B2 Vp 4.5 8.0 5.73 3
25 HD 66765 B1/B2 V 5.3 7.5 1.61 100
26 HD 64740 HR 3089 B1.5 Vp 6.3 11 1.33026 160
27 ALS 15956 Col 228 30 B1.5 V 9.1 11
28 ALS 9522 NGC 6611 W601 B1.5 Ve HeBe 6.4 10 190
29 HD 36982 LP Ori B1.5 Vp 2.5 2.2 80
30 HD 37017  V 1046 Ori B1.5-2.5 IV-Vp SB2 3.9 7.2 0.90119 90
31 HD 37479  Ori E B2 Vp 3.9 5.0 1.1908 170
32 HD 149277  B2 IV/V SB2 7.0 17 15
33 HD 184927 V 1671 Cyg B2 Vp 4.3 5.5 9.530 14
34 HD 37776  V 901 Ori B2 Vp 3.8 5.5 1.538756 95
35 HD 136504   Lup B2 IV-V SB2, Cep 5.3 8.6 42
36 HD 156424 B2 V 4.8 8.5 15
37 HD 156324 B2 V 5.1 9.4 60
38 HD 121743  Cen B2 IV  Cep 4.7 8.0 80
39 HD 3360  Cas B2 IV SPB 5.9 8.3 5.37045 17
40 HD 186205  B2 Vp 4.9 7.4 5
41 HD 67621 B2 IV 4.1 6.2 3.59 20
42 HD 200775  V 3780 Cep B2 Ve SB2, HeBe 10 10 4.328 26
43 HD 35912 HR 1820 B2 V 4.4 7.2 0.89786  12
44 HD 66522 B2 III 4.6 7.6  10
45 HD 182180 HR 7355 B2 Vn 3.7 6.0 0.5214404 310
46 HD 55522 HR 2718 B2 IV/V 3.3 5.5 2.729 70
47 HD 142184 HR 5907 B2 V 3.1 5.5 0.50828 290
48 HD 58260  B3 Vp 9.5 9.5  12
49 HD 36485   Ori C B3 Vp SB2 4.5 7.1 1.47775 32
50 HD 208057  16 Peg B3 V SPB 5.5 7.1 1.441 104
51 HD 306795 NGC 3766 MG170 B3 V 4.1 4.3 65
52 HD 25558  40 Tau B3 V SPB 3.9 5.5 28
53 HD 35298  B3 Vw 5.5 5.6 1.85336 260
54 HD 130807  Lup B5 3.5 5.7 25
55 HD 142990  V 913 Sco B5 V 3.1 5.7 0.97907 125
56 HD 37058  V 359 Ori B3 VpC 5.6 6.6 14.61 25
57 HD 35502  B5 V SB2 5.7 5.7 0.85 80
58 HD 176582 HR 7185 B5 IV 3.6 4.7 1.581984 105
59 HD 189775 HR 7651 B5 V 5.3 5.5 2.6048 85
60 HD 61556  HR 2949 B5 V 2.8 2.9 1.9093 70
61 HD 175362  Wolff’s star B5 V 5.8 5.3 3.6738 35
62 HD 105382  HR 4618 B6 III 3.6 4.8 1.285 90
63 HD 125823 a Cen B7 IIIp 3.6 4.7 8.812 15
64 HD 36526 V1099 Ori B8 Vp 2.4 2.0 1.5405
Notes in Appendix.
(5) Single- or double-lined spectroscopic binary (SB1-2), slowly pulsating B-type star (SPB),  Cep-type pulsator ( Cep), Herbig Be star (HeBe).
Parameters determined from modern spectral modelling.
Luminosity derived from our photometric calculations with bolometric correction.
Luminosity derived from SED fitting with Chorizos.
Higher multipole components.
Table 1: Continued
ID Star Ref. ID Star Ref.
(1) (2) (3) (1) (2) (3)
1 HD 148937 Wade et al. (2012b) 32 HD 149277  Bagnulo et al. (2006)
A. Fullterton (priv. com.) Landstreet et al. (2007)
Nazé et al. (2012b) 33 HD 184927 Wade et al. (1997)
2 CPD -28 2561 Barba et al. (MiMeS in prep) Yakunin et al. (MiMeS in prep)
3 HD 37022 Simón-Díaz et al. (2006) 34 HD 37776 Landstreet et al. (2007)
Stahl et al. (2008) Kochukhov et al. (2011)
Wade et al. (2006) Mikulášek et al. (2011)
Walborn & Nichols (1994) Shultz et al. (MiMeS in prep)
Stelzer et al. (2005) Shore & Brown (1990)
4 HD 191612 Wade et al. (2011b) 35 HD 136504  Uytterhoeven et al. (2005)
A. Fullerton (priv. com.) Shultz et al. (2012)
Nazé et al. (2007) Hubrig et al. (2009)
5 NGC 1624-2 Wade et al. (2012a) 36 HD 156424 E. Alecian (MiMeS in prep)
6 HD 47129 Linder et al. (2008) 37 HD 156324 E. Alecian (MiMeS in prep)
Grunhut et al. (2012b) 38 HD 121743 Wolff (1990)
7 HD 108 Martins et al. (2010) E. Alecian (MiMeS in prep)
Marcolino et al. (2012) Grillo et al. (1992)
Linder et al. (2006) 39 HD 3360 Neiner et al. (2003)
8 ALS 15218 Gagné et al. (2011) Oskinova et al. (2011)
Nazé et al. (2012a) 40 HD 186205 Zboril & North (2000)
Nazé et al. (2011) J. Grunhut (MiMeS priv. com.)
9 HD 57682 ‘Grunhut et al. (2009) 41 HD 67621 Alecian et al. (MiMeS in prep)
Grunhut et al. (2012c) 42 HD 200775 Alecian et al. (2008a)
10 HD 37742 ‘Bouret et al. (2008) Hamaguchi et al. (2005)
Kaper et al. (1996) 43 HD 35912 Simón-Díaz (2010)
Raassen et al. (2008) Bychkov et al. (2005)
11 HD 149438  ‘Simón-Díaz et al. (2006) 44 HD 66522 Zboril et al. (1997)
Donati et al. (2006b) Leone et al. (1997)
Mewe et al. (2003) E. Alecian (MiMeS in prep)
12 HD 37061  ‘Simón-Díaz et al. (2011) 45 HD 182180 Rivinius et al. (2012)
Petit et al. (2008) 46 HD 55522  Briquet et al. (2004)
Stelzer et al. (2005) Briquet et al. (2007)
13 HD 63425 Petit et al. (2011) 47 HD 142184 Grunhut et al. (2012a)
14 HD 66665 ‘Petit et al. (2011) Oskinova et al. (2011)
15 HD 46328 Fourtune-Ravard et al. (2011) 48 HD 58260 Bohlender (1989)
Oskinova et al. (2011) Cidale et al. (2007)
16 ALS 8988 ‘Alecian et al. (2008b) Pedersen (1979)
Wang et al. (2008) Shore & Brown (1990)
17 HD 47777 E. Alecian (priv. com.) 49 HD 36485 Leone et al. (2010)
Nazé (2009) Shore & Brown (1990)
18 HD 205021 ‘Donati et al. (2001) 50 HD 208057 ‘Chauville et al. (2001)
Lefever et al. (2010) Henrichs et al. (2009)
Catanzaro (2008) 51 HD 306795 ‘McSwain (2008)
Favata et al. (2009) McSwain et al. (2008)
19 ALS 15211 Gagné et al. (2011) 52 HD 25558  ‘Lefever et al. (2010)
Nazé et al. (2012a) 53 HD 35298  ‘Landstreet et al. (2007)
Nazé et al. (2011) Bychkov et al. (2005)
20 HD 122451  ‘Ausseloos et al. (2006) Yakunin et al. (2011)
Alecian et al. (2011) 54 HD 130807 ‘Alecian et al. (2011)
H. Henrichs (priv. com.) 55 HD 142990 ‘Cidale et al. (2007)
Raassen et al. (2005) Bychkov et al. (2005)
21 HD 127381 Henrichs et al. (2012) Shore et al. (2004)
22 ALS 3694 Bagnulo et al. (2006) 56 HD 37058 Glagolevskij et al. (2007)
Landstreet et al. (2007) Pedersen (1979)
Huang & Gies (2006) Bychkov et al. (2005)
23 HD 163472 ? Ramírez et al. (2004)
Neiner et al. (2012) 57 HD 35502 ‘Landstreet et al. (2007)
C. Neiner (priv. com.) Romanyuk & Kudryavtsev (2008)
Oskinova et al. (2011) Bohlender et al. (in prep)
24 HD 96446 Neiner et al. (2012b) Grillo et al. (1992)
Shore & Brown (1990) 58 HD 176582 ‘Bohlender & Monin (2011)
25 HD 66765 Cidale et al. (2007) 59 HD 189775 ‘Lyubimkov et al. (2002)
Alecian et al. (MiMeS in prep) Bohlender et al. (priv. com.)
26 HD 64740 Bohlender & Landstreet (1990) 60 HD 61556 ‘Rivinius et al. (2003)
Shore & Brown (1990) Shultz et al. (in prep)
Peralta et al. (MiMeS in prep) 61 HD 175362 Leone & Manfre (1997)
Drake et al. (1994) Bychkov et al. (2005)
27 ALS 15956 Bagnulo et al. (2006) Shore et al. (2004)
Nazé et al. (2011) Grillo et al. (1992)
28 ALS 9522 Alecian et al. (2008b) 62 HD 105382 Briquet et al. (2001)
Guarcello et al. (2012) Alecian et al. (2011)
29 HD 36982  Wolff et al. (2004) 63 HD 125823 Bohlender et al. (2010)
Petit & Wade (2012) 64 HD 36526 Landstreet et al. (2007)
Stelzer et al. (2005) Bychkov et al. (2005)
30 HD 37017  Bolton et al. (1998) Romanyuk & Kudryavtsev (2008)
Bohlender et al. (1987)
Shore & Brown (1990)
Oskinova et al. (2011)
31 HD 37479 Hunger et al. (1989)
Townsend et al. (2010)
Oksala et al. (2012)
Shore & Brown (1990)
Sanz-Forcada et al. (2004)
Stellar parameters, Rotational parameters, Magnetic field parameters
H proxy, UV proxy, X-ray proxy
Table 2: Continued

2.1 Sample selection

Magnetic fields in hot stars can be detected through the circular polarisation induced in spectral lines by the Zeeman effect, using various types of instruments. The bulk of cooler magnetic ApBp stars were generally detected with first-generation photo-polarimeters, measuring for example the degree of polarisation in the wings of a Balmer line (e.g. Borra & Landstreet, 1980).

However, apart from a few strongly magnetic He-strong stars such as  Ori E, the bulk of hot magnetic OB stars were detected with second generation instruments, such as the low resolution ( a few thousands) spectropolarimetry optics used in FORS 1 and 2 (VLT) and the high resolution ( a few tens of thousands) spectropolarimeters MUSICOS, ESPaDOnS, Narval and HARPSpol at the TBL, CFHT, TBL and ESO-3.6m, respectively. These two classes of instruments differ in that low resolution spectropolarimeters are only sensitive to the disk-integrated, brightness-weighted longitudinal field component, whereas high-resolution instruments can probe field configurations through the rotationally induced Doppler shifts within the resolved line profiles (see Donati & Landstreet, 2009; Petit, 2011).

We use the existing compilations of ApBp stars (e.g. Bychkov et al., 2005; Landstreet et al., 2007; Romanyuk & Kudryavtsev, 2008) as well as an exhaustive review of the literature to identify hot stars with confirmed field detections, which we complement with new detections from the MiMeS project.

Some concerns have recently been raised about claimed magnetic detections (usually near the level) obtained with the FORS instruments that were not reproduced with other high-resolution instruments (see Silvester et al., 2009; Shultz et al., 2012). Bagnulo et al. (2012) performed an in-depth study of the complete set of FORS circular polarisation measurements in the ESO archive, exploring the effect of various data reduction procedures and carefully considering all known sources of uncertainties. Using their new prescription for FORS data analysis, most of the claimed marginal detections were found to have very low significance, in agreement with the results from high-resolution instruments. They also provided updated longitudinal field values and new magnetic detection statuses for stars that were reported magnetic in the literature at the level. We therefore base our selection on these new detection statuses for stars that were only detected with the FORS instruments.

It is worth noting that stars with chemical abundance peculiarities can have effective temperatures that do not reflect their spectral types, as the latter is determined from spectral morphology. In particular He-strong/weak stars, which form the majority of the cooler part of our sample, are identified by their unusually strong/weak He lines, lines that are the basic means to classify B-type stars. Given that photometric/spectral effective temperature determinations are not always readily available, it is therefore difficult to assess the completeness of our sample at the low temperature boundary. We therefore consider all magnetic stars with spectral type B5 and earlier, as well as additional stars of later spectral type known to have effective temperatures greater than 16 kK. We believe the sample at these temperatures (and above) to be a substantially complete representation of the currently known hot magnetic stars.

Although we consider a detailed review of the large sample of stars evaluated for inclusion in Table 1 beyond the scope of this work, two noteworthy objects require a brief mention. The first of these is the Be star  Ori, reported to be magnetic by Neiner et al. (2003c) based on MuSiCoS longitudinal field measurements. Recently, Neiner et al. (2012a) have retracted this claim based on new ESPaDOnS and Narval measurements. The second is  Ori A, reported to be magnetic by Bouret et al. (2008). While no single observation of this star yields a significant magnetic detection, overall we consider the evidence presented by Bouret et al. (2008) to be sufficiently compelling that we retain this star in our list. Note that  Ori A occupies a unique position in the magnetic confinement-rotation diagram (see § 3).

2.2 Physical parameters

Effective temperatures and surface gravities (columns 6 and 7 of Table 1) were retrieved from the literature. An superscript in column (6) indicates stellar parameters that were determined by modern spectral modelling, with NTLE model atmospheres such as cmfgen, tlusty or fastwind for the hotter stars, or such as LTE atlas models with the polarised radiative transfer code zeeman for the cooler stars (Hillier & Miller, 1998; Lanz & Hubeny, 2003; Puls et al., 2005; Kurucz, 1979; Landstreet, 1988; Wade et al., 2001). For the other stars, temperatures and gravities were generally derived from photometry combined with spectral type calibrations. Some details are given in the notes of Appendix A in cases where significant discrepancies were found in the literature values or when we had to estimate from the luminosity class.

When modern spectral modelling is available, we use the literature value for the luminosity, radius and mass (columns 8, 9 and 10). The luminosity is generally obtained through a distance estimate and photometry, and the spectroscopic mass is derived from the surface gravity and radius, unless a better estimate is available from a binary orbit.

For most of the remaining stars, marked with superscript or in column (8), we derive the luminosity from photometry (see §2.2.1) using tabulated bolometric corrections, or using the spectral energy distribution (SED) fitting code Chorizos (Maíz-Apellániz, 2004) for stars with sufficient photometric data.

In Figure 1, we locate the magnetic OB stars on the HR diagram. The symbol shapes represent the O-type stars (circles), B-type stars hotter than 22 kK (squares), those between 22 kK and 19 kK (triangles) and those that are cooler than 19 kK (pentagons), and known Herbig Be stars (HeBe; diamonds). The luminosity classes are colour coded. The labels refer to the identification numbers in column (1) of Table 1.

Figure 1: Location of the magnetic stars in the HR diagram. The labels refer to ID sequence number listed in column (1) of Table 1. The various symbol shapes represent effective temperature ranges and colours denote luminosity classes, as indicated in the legend. The shaded region shows the main sequence, from ZAMS to TAMS (from the galactic evolutionary tracks of Brott et al., 2011). The grey line shows the mid-way main sequence with spectral types calibrations from Martins et al. (2005) for O-type stars and de Jager & Nieuwenhuijzen (1987) for B-type stars.

The position of the spectral types, from the calibrations of Martins et al. (2005) for the O-type stars and de Jager & Nieuwenhuijzen (1987) for the B-type stars, are indicated on the dark grey line that runs approximately mid-way between the zero-age main sequence and the terminal-age main sequence; the main sequence itself is shown by the light grey shaded area (from the galactic evolutionary tracks of Brott et al., 2011).

Luminosity derivation

For each star in our sample without modern spectral modelling, Table 3 gathers visual magnitudes and colours (columns 3-5) in the Johnson system, from the compilations of Mermilliod (2006) and Reed (2005)5. We also provide magnitudes (columns 6-9) from the NOMAD catalog (Zacharias et al., 2005), which will be used below for SED fitting with Chorizos.

ID Star ()
 mag  mag mag  mag  mag  mag  mag
(1) (2) (3) (4) (5) (6) (7) (8) (9)
20 HD 122451 

22
ALS 3694

25
HD 66765

27
ALS 15956

29
HD 36982

31
HD 37479

32
HD 149277 

35
HD 136504 

36
HD 156424

37
HD 156324

38
HD 121743

40
HD 186205 

41
HD 67621

43
HD 35912

44
HD 66522

48
HD 58260 

49
HD 36485 

50
HD 208057 

51
HD 306795

52
HD 25558 

53
HD 35298 

54
HD 130807

55
HD 142990 

56
HD 37058 

57
HD 35502 

59
HD 189775

60
HD 61556 

61
HD 175362 

62
HD 105382 

63
HD 125823

64
HD 36526

Because of their brightness, these stars have uncertainties of  mag according to the 2mass specifications.
Table 3: Photometry of magnetic stars without modern spectral modelling (§ 2.2.1).

For all these stars, we derive the luminosity using bolometric corrections () and extinction () evaluated from the intrinsic colour . The results are compiled in Table 4. The distance modulus (; column 4) is estimated using either Hipparcos parallax measurements or a distance estimate from an association with a stellar cluster. The Hipparcos distances are corrected for Lutz-Kelker-type effects (Lutz & Kelker, 1973) using the technique described by Maíz-Apellániz (2001; 2005) updated to the new reduction of the Hipparcos data (van Leeuwen, 2007) by Maíz Apellániz et al. (2008).

ID Star
cgs mag  mag  mag  mag mag mag
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
20 HD 122451 

22
ALS 3694

25
HD 66765

27
ALS 15956

29
HD 36982

31
HD 37479

32
HD 149277 

35
HD 136504 

36
HD 156424

37
HD 156324

38
HD 121743

40
HD 186205 

41
HD 67621

43
HD 35912

44
HD 66522

48
HD 58260 

49
HD 36485 

50
HD 208057 

51
HD 306795

52
HD 25558 

53
HD 35298 

54
HD 130807

55
HD 142990 

56
HD 37058 

57
HD 35502 

59
HD 189775

60
HD 61556 

61
HD 175362 

62
HD 105382 

63
HD 125823

64
HD 36526

From the SB2 analysis of Ausseloos et al. (2006).
Distance estimates from associations with stellar clusters (Hipparcos otherwise).
Table 4: Luminosity determination based on bolometric correction and extinction from intrinsic colours (§ 2.2.1).

The theoretical and (columns 5 and 6) are determined from a smooth interpolation of the grids provided by Martins et al. (2005), and Martins & Plez (2006) for the O-type stars and Lanz & Hubeny (2007) for the B-type stars. We use an extinction to derive the extinction (column 7). The absolute visual magnitude (), the bolometric magnitude () and the luminosity [] are given in columns 8 to 10.

With a typical uncertainty of 2 000 K in and 0.3 dex in , we estimate an uncertainty of 0.2 and 0.02 mag in and , respectively. Given the wide range of often encountered in the literature for OB stars, we adopt a conservative error in of 0.25 mag. In most cases, , and contribute equally to the uncertainty, leading to 0.2-0.3 dex for the luminosity. In five cases (ID: 36, 37, 40, 53 and 57) the luminosity error estimate from the bolometric correction method is more than 0.4 dex, given the large uncertainty in distance.

ID Star
mag mag cgs
(1) (2) (3) (4) (5) (6) (7)
25 HD 66765

27
ALS 15956

35
HD 136504